Cold sensitive and heat sensitive mutants of Saccahromyces cerevisiae which become reversibly defective in mitochondrial function at the nonpermissive temperature (18 degrees C or 37 degrees C) will be used in genetic and biochemical analyses to elucidate the sequence of events involved in the growth and differentiation of mitochondrial membranes. The broad goals of the proposed research program are: 1) to determine the extent to which membrane biogenesis depends on an ordered pathway of biosynthetic and assembly steps: 2) to ascertain the cellular location of the genetic determinants controlling these processes; and 3) to define at the molecular level the sequence of biochemical events leading to the formation of fully functional membrane-bound enzyme complexes of the inner mitochondrial membrane. Strains will be constructed which contain at least one cold sensitive and one heat sensitive mutation in the same haploid cell. By analyzing these strains for the appearance (or disappearance) of membrane function during shifts from one nonpermissive temperature to the other, it will be possible to test for the existence of an ordered sequence of events in membrane biogenesis and, if such a pathway can be demonstrated, to order the mutations according to the relative position in the pathway where the lesions are expressed.